The invention relates to a digital signal recording system, and more particularly, to a system in which digital signals are modulated in a manner to permit a self-clocking and a peak detection.
When recording digital signals on a carrier, they may be directly recorded without change in their form, or alternatively they may be modulated before they are recorded. When the recording medium is a magnetic tape or disc, the latter procedure is commonly employed. A variety of recording systems are known in this instance. Typical techniques include a return-to-zero (RZ) scheme, a non-return-to-zero (NRZ) scheme, a non-return-to-zero inverted (NRZI) scheme, a frequency modulation (FM) scheme, a phase modulation (PM) scheme, and a modified frequency modulation (MFM) scheme. Each of these schemes is well known and will not be specifically dealt with. However, it is to be noted that they selectively exhibit various features relating to the self-clocking capability, recording density, frequency band, peak detection capability or the like. It is therefore important to determine a most suitable recording system when recording digital signals, taking into consideration the kind of a carrier being used and the signals to be recorded.
However, it is found that with certain carriers, inconvenience may be caused when recording signals according to a known recording system. Specifically, this occurs when signals are recorded on a carrier in the form of grooves formed in physically deformable surface areas. A system which employs such carrier to reproduce stored signals therefrom is disclosed in West Germany Laid-Open Patent Application (DE-OS) No. P 2 024 539 or "JOURNAL OF THE SMPTE" Vol. 8, pages 303-304, issued April 1972. A signal is recorded in the carrier in the form of a groove in the carrier surface, and during the reproduction, a scanner runs along a given track of the carrier. A scanning surface of the scanner is substantially fixed in position in the direction of the reaction force produced by the carrier surface so that a deformed portion which lies therebelow exerts a pressure upon the scanning surface, with a change in such pressure or force being converted into an electrical quantity. With this recording and reproducing system, it is possible to derive from the carrier, in the form of a disc, frequencies which are substantially higher than the frequency band usually available from a common hard record disc, so that it can be advantageously used for the recording of a wide band video signal in addition to the acoustic signal. If digital signals can be recorded on such disc, the voice signal can then be recorded according to a pulse code modulation scheme which enables the reproduction with a high fidelity, with the possibility of recording a multi-channel voice signal as a time division multiplex PCM signal. However, the prior art failed to achieve such result when utilizing the known recording schemes. The reason therefor will be discussed below with reference to FIG. 1.
FIG. 1 shows a known reproducing unit of pressure scanning type, which includes a mechanical-electrical transducer 1 comprising a piezoelectric ceramic material, and a slide tip 2 of diamond which is secured to the transducer 1 by means of adhesive 3. The tip 2 is adapted to contact the surface of a carrier disc 4, in which a deformation corresponding to a time varying signal is formed in the form of a groove. As the disc 4 moves in a direction indicated by an arrow, the unit is subjected to a change in pressure which is related to the stored signal in the carrier 4. By scanning the carrier surface such as that shown in FIG. 1, there can be reproduced a sinusoidal wave as shown in FIG. 2.
FIG. 3(a) shows a digital signal comprising thirteen bits 1001110100011 which may be directly recorded in the carrier 4. FIG. 3(b) shows an output waveform which may be reproduced from the recorded signal by the reproducing unit. Where adjacent bit cells have binary one in succession as indicated in cells 4 and 5 and cells 12 and 13, the bits cannot be reproduced except for the last one in the succession. As a consequence, a digital signal must be recorded in the carrier by recording a modulated version thereof. Where a signal is recorded on a single track of the disc, an arrangement must be made to enable a self-clocking operation during the reproduction. Since jitter is involved with a change in the number of revolutions of the disc, it is necessary to provide clock pulses which include components corresponding to the jitter contained in the reproduced signal in order to avoid the absence of clock information over a prolonged portion of the reproduced signal. Also, since it is difficult to discriminate information based on the amplitude of the reproduced signal, it is desirable to adopt a peak detection system to discriminate information on the basis of the presence or absence of a peak, which can be detected by differentiation of the output waveform since the gradient is null at the peak.
Three recording systems including FM, PM and MFM are available to satisfy the need of a disc in general including a magnetic disc. FIGS. 3(e), (f) and (g) show waveforms which result from the modulation of the digital signal shown in FIG. 3(a) according to FM, PM and MFM schemes, respectively. These schemes do not lend themselves or are dissatisfactory to a certain kind of discs as mentioned above. It will be understood that FIG. 3(c) shows clock pulses while FIG. 3(d) shows bit pulses. When a pressure scanning, reproducing unit is used, the slide tip causes an elastic deformation of hills formed in the disc surface, which causes a large change in the electrical signal to be produced during the movement of the carrier when it is released from an edge of the slide tip which is formed at an angle of nearly 90.degree.. As a consequence, information must be consistently present at either one of the rising or falling end of the pulses. By observation of FIGS. 3(e), (f) and (g) where the position of information is conveniently indicated by arrows, it will be readily apparent that the FM and MFM schemes are inadequate for the reasons mentioned above. In this respect, it is seen that the PM scheme is favourable in that it provides a consistent positioning of information, but it suffers from another disadvantage. Specifically, representing the period of a clock pulse by T.sub.0, the pulse width of PM wave will be of two kinds, one having T.sub.0 and another T.sub.0 /2. This results in a displacement of the signal peak from an accurate position in time if the signal transmission system includes a portion of a reduced frequency band. The consequence is the introduction of extraneous signal or a drop-out of necessary signal when the detected peak signal is ANDed with the clock pulse.